1. Φ 3. Transformer Models. Turns ratio 7200:240 (30 : 1)
(approx. same amount of copper in each winding) Rs
jXs
Ideal
Transformer
7200:240V Rm
jXm
7200V V
Prof. Mack Grady, TAMU Relay Conference Tutorial Topic 3, March 31, 2015

2. Φ 3. Transformer Models, cont. Open Circuit Test Ioc + Voc - ~ V R ||Rs
jXs +
Ideal Rm
jXm
Transformer
Voc
7200:240V -
7200V V
Turns ratio 7200:240 Φ
Open circuit test: Open circuit
the 7200V -
side, and apply
240V to the 240V -
side. The
winding currents are small, so
the series terms are negligible. ~ V R || jX = oc m m ~ I oc
Prof. Mack Grady, TAMU Relay Conference Tutorial, Topic 3, March 31, 2015

3. Φ 3. Transformer Models, cont. Short Circuit Test Isc + Vsc -
Short circuit test: Short circuit the 240V-side, and raise the 7200V-side voltage to a few percent of 7200, until rated current flows. There is almost no core flux so the magnetizing terms are negligible. +
Vsc -
Isc Rs
jXs
Ideal Rm
jXm
Transformer
7200:240V
7200V V
Turns ratio 7200:240 Φ
Prof. Mack Grady, TAMU Relay Conference Tutorial, Topic 3, March 31, 2015

4. 3. Transformer Models, cont.Rs
jXs
Ideal
Transformer Rm
jXm
X / R Ratios for Three -
Phase Transformers •
345kV to 138kV, X/R = 10
Substation transformers (e.g., 138kV to 25kV or 12.5kV, X/R = 2, X = 12%
25kV or 12.5kV to 480V, X/R = 1, X = 5%
480V class, X/R = 0.1, X = 1.5% to 4.5%
Prof. Mack Grady, TAMU Relay Conference Tutorial, Topic 3, March 31, 2015

5. 3. Transformer Models, cont.
1. Given the standard percentage values below for a 125kVA transformer, determine the R’s and X’s in the diagram, in Ω.
If the R’s and X’s are moved to the 240V side, compute the new Ω values.
If standard open circuit and short circuit tests are performed on this transformer, what will be the P’s and Q’s (Watts and VArs) measured in those tests?
Load loss
Single Phase Transformer. Percent values on transformer base.
Winding 1
kV = 7.2, kVA = 125
Winding 2
kV = 0.24, kVA = 125
%Imag = 0.5
%Load loss = 0.9
%No-load loss = 0.2
%Xs = 2.2 Xs
No-load loss
Magnetizing current Rs
jXs
Ideal
Transformer
7200:240V Rm
jXm
7200V V
Prof. Mack Grady, TAMU Relay Conference Tutorial, Topic 3, March 31, 2015

6. Annual Feeder Loss Components
3. Transformer Models, cont.
EPRI Study, Distribution Feeder Loss Example
Annual energy loss = 2.40% •
Largest component is transformer no-load •
loss (45% of the 2.40%)
Transformer No-
Load
45%
Transformer Load 8%
Primary Lines
26%
Secondary Lines
21%
Annual Feeder Loss Components
Modern Distribution Transformer:
Load loss at rated load (I2R in conductors) = 0.75% of rated transformer kW.
No load loss at rated voltage (magnetizing, core steel) = 0.2% of rated transformer kW.
Magnetizing current = 0.5% of rated transformer amperes

7. Y - 3. Transformer Models, cont.
A three-phase transformer can be three separate single-phase transformers, or one large transformer with three sets of windings Y -
N1:N2 Rs
jXs
Ideal
Transformer
N1 : N2 Rm
jXm
Wye-Equivalent One-Line Model A N •
Reflect to side 2 using individual transformer
turns ratio N1:N2 ,
Standard 345/138kV autotransformers,
GY-GY
have a tertiary 12.5kV Δ
winding to permit
circulating 3 rd
harmonic current
Prof. Mack Grady, TAMU Relay Conference Tutorial, Topic 3, March 31, 2015

8. Δ - Δ 3. Transformer Models, cont.
For Modeling a Delta-Delta Connection, Convert the Transformer to Equivalent Wye-Wye
Wye-Equivalent One-Line Model Rs
jXs A
Ideal 3 3
N1:N2
Transformer Rm
jXm 3 3 N 1 N 2 : N 3 3
N1:N2 •
Convert side 1 impedances from delta to
equivalent wye •
Then reflect to side 2 using individual
transformer turns ratio N1:N2
N1:N2 Δ - Δ
Prof. Mack Grady, TAMU Relay Conference Tutorial, Topic 3, March 31, 2015

9. Δ - Y 3. Transformer Models, cont.
For Modeling a Delta-Wye Connection, Convert the Transformer to Equivalent Wye-Wye Δ - Y
N1:N2
Ideal
Transformer 3 Rs 2 : 1 N
jXs Rm
jXm A
Wye-Equivalent One-Line Model
Convert side 1 impedances from delta to wye
Then reflect to side 2 using three-phase line-to-line turns ratio
Has 30° degree phase shift due to line-to-neutral to line-to-line relationship. ANSI standard requires the transformer to be labeled such that high-voltage side leads the low-voltage side by 30° .
Prof. Mack Grady, TAMU Relay Conference Tutorial, Topic 3, March 31, 2015

10. Y - Δ 3. Transformer Models, cont.
For Modeling a Wye-Delta Connection, Convert the Transformer to Equivalent Wye-Wye Y - Δ
N1:N2
Ideal
Transformer 3 2 : 1 N
jXs
Wye-Equivalent One-Line Model A Rs Rm
jXm N
Reflect to side 2 using three-phase bank line-to-line turns ratio
Has 30° degree phase shift due to line-to-neutral to line-to-line relationship. ANSI standard requires the transformer to be labeled such that high-voltage side leads the low-voltage side by 30°
Thus, for all configurations, equivalent wye-wye transformer ohms can be reflected from one side to the other using the three-phase bank line-to-line turns ratio
Prof. Mack Grady, TAMU Relay Conference Tutorial, Topic 3, March 31, 2015